Heating elements such as those disclosed in U.S. Pat. No. 4,616,125 or U.S. Pat. No. 5,849,137 include two conductive films embedding a positive temperature coefficient (PTC) layer between them. Heating elements comprising conductive films are not very air-permeable and lack flexibility.
U.S. Pat. No. 5,002,336 discloses a seat with air conditioning means. It has an air-distributing layer made up of spiral springs. The air conditioning of the seat, however, is based on feeding fully conditioned air into the air distributing layer and distributing it uniformly in the seat. Such a system is bulky and lacks the ability to accurately and rapidly heat or condition the seating surface.
DE 1,993,174 discloses a heating element whose heating conductor is composed of film segments arranged in a labyrinth-fashion. In that case, the current heats by conventional resistance heating, while it flows labyrinth-fashion along the plane of the heating elements. Such heating elements are not very air-permeable and require an external temperature-regulating means.
Accordingly, there exists a need for an improved areal heating element that is highly flexible, air permeable, robust and responsive.